136 THE PHYSIOLOGY OF TWINNING 



result of the twinning relation, one twin tends to gain a 

 physiological ascendancy over the other, to the slight 

 or very great detriment of the latter. As to the extent 

 to which one twin may harm the other during pregnancy 

 there is considerable difference of opinion. 



Spaeth (i860) was probably the earliest observer to 

 study this problem. He was chiefly interested in the 

 question whether the interinfluence between fetuses was 

 greater in one-egg than in two-egg twins. His material 

 consisted of sixty-five pairs of new-born twins and their 

 embryonic membranes. Whether the twins were the 

 products of a single egg or of two eggs was judged by the 

 relations they bore to the placenta and the other mem- 

 branes, especially the amnion. A comparison between 

 one-egg and two-egg twins showed that twins of both 

 kinds are nearly always rather markedly unequal in 

 size and in body length. In only three cases out of the 

 sixty-five examined were the twins even of similar size 

 and length. One of these cases of striking similarity, 

 judged by their possession of a common placenta, com- 

 mon chorion, and common amnion, was undoubtedly a 

 case of one-egg twins. The other two cases of close 

 similarity were in two-egg twins. There was no evi- 

 dence that the twins of either type had any definite 

 physiological effect upon each other and Spaeth concludes 

 that, although twins are so closely associated during 

 pregnancy, they maintained a high degree of independ- 

 ence. In only one respect does he see evidences of inter- 

 influence: in the occurrence of situs inversus viscerum. 

 In a number of cases he noticed in one of the twins a 

 reversed symmetry of stomach, heart, liver, and other 

 more or less asymmetrical organs. As the whole ques- 



